A pro-Russian rebel leader has told Russian TV that thousands of Russian nationals have joined his forces to fight against the Ukrainian military.

The prime minister of the self-declared Donetsk People’s Republic, Alexander Zakharchenko, said that the Russians amongst his ranks were volunteers made up of former Russian military personnel as well as current personnel on leave. He said that the Russian volunteers saw joining the pro-Russian separatists as their “duty”.

These comments come as Ukrainian officials claim that Russian forces have crossed the border and are actively supporting the rebels as they move towards the key port of Mariupol.

Ukrainian President Petro Poroshenko blamed “Russian troop deployments” in the east of the country for cancelling a scheduled official trip to Turkey.

Moscow denies that they are arming or militarily supporting the rebels, and claims that the ten Russian soldiers captured by Ukrainian forces within Ukraine “accidentally” crossed the border.

The rapidly deteriorating situation in eastern Ukraine has damaged hopes that the Minsk summit earlier this week could bring an end to five months of conflict.

More than 2,000 people, mostly civilians, have been killed in eastern Ukraine since April, with more than 300,000 people fleeing the region to other parts of Ukraine and over the border into Russia.